This photorefractive device or photorefractive structure can be used in optical telecommunications applications, especially in optical switching applications. In a disposition in the form of a phase conjugation double mirror, this structure is able to self-focus a luminous beam between two monomode optical fibers. In this respect, the following documents can be consulted:
(1) N. Wolfer, P. Gravey, J. Y. Moisan, C. Laulan, J. C. Launay, Optics Commun., 73, 351 (1989). PA1 (2) G. Picoli, P. Gravey, J. E. Viallet, French patent Application No. 91 07528 of 19 Jun. 1991--see also the document WO 92/22847, PA1 (3) D. S. Chemla, T. C. Damen, D. A. B. Miller, A. C. Gossard, W. Wiegmann, Appl. Phys. Lett., 42, 864 (1983), PA1 (4) E. Bigan, L'Echo des Recherches, 149, 29 (1992) PA1 (5) D. D. Nolte, D. H. Olson, G. E. Doran, W. H. Knox, A.M. Glass, "Resonant photorefractive effect in semi-insulating multiple quantum wells", J. Opt. Soc. Am, B, 7, 2217-2225 (1990). PA1 (6) Partovi, Glass, Olson, Zydzik, Short, Feldman, Austin, Opt. Lett., 17, 655 (1992).
The embodiment of a flat matrix of photorefractive cells makes it possible to connect networks of optical fibers.
The present invention authorizes the implementation of a significant photorefractive effect with the aid of thin films or layers of semi-conductor materials. In addition, the device of the invention preferably is able to have a large use or repetition frequency (Writing and erasion).
Note that the operating principle of a photorefractive material resides in the marking, inside this material, of a grating of refraction indices which are relatively large so as to effectively diffract incident optical waves (reading radiation). This grating of refraction indices derives from a network of electric charges optically excited via the interference of two luminous beams constituting the marking radiation and generally being laser beams.
In known types of photorefractive materials, this network of electric charges is preserved via the trapping of these charges on deep centers. This type of photorefractive material ought to have the following characteristics: it needs to be semi-insulating in the absence of light; it needs to exhibit an electro-optical effect as powerful as possible so as to exhibit significant refraction index variations; and it needs to have a sufficient number of deep centers so as to produce local modifications of the electric field.
There is currently no material able to optimize both the electro-optical function and the trapping function effected by deep centers. So as to resolve this drawback, a device described in the following document:
has been embodied so as to separate these two functions. In this known type of photorefractive device, a quantum well material possessing a high electro-optical coefficient is sandwiched between two semi-conductor layers having opposing dopings. Thus, a significant photorefractive effect is obtained with an active layer whose thickness is close to 1 micrometer. On the other hand, the deep centers of this known device have a low coefficient of optical absorption (10 cm.sup.-1 for a concentration of about 10.sup.17 cm.sup.-2), which requires a significant incident optical power since merely one thousandth of the available power is absorbed in the layers which are doped by the deep centers and whose thickness is about 1 micrometer.
Moreover, it is well known that it is possible to embody extremely effective electro-optical devices by applying an electric field to a multiple quantum well or to a super lattice. In this respect, it is possible to consult the following document:
which describes electro-optical devices based on the confined Stark effect. The following document may also be consulted:
which describes electro-optical devices using super-grating Wannier Strak modulators.
The method is also known on how to transform structures with quantum wells into photorefractive structures by using implantation or irradiation so as to create deep centers in these structures. In this respect, the following document may be consulted:
Thus a photorefractive effect is obtained but this effect does not use (in the GaAs system) the advantages of the confined Stark effect as the electric field associated with the network of electric charges is parallel to the planes of the quantum wells. These types of structures result in obtaining low refraction index variations.
Higher refraction index variations have been obtained in quantum multiple wells with a II-VI semi-conductor materials base in a configuration where the electric field may be applied perpendicular to the layers of these multiple quantum wells. In this respect, the following document may be consulted:
However, the photorefractive devices obtained in this way require the use of an alternative polarization as the electric charges are not stored correctly adjacent to the contact layers of these devices.